In vehicles, in particular utility vehicles, with a pneumatic brake system, axle modulators can be electronically actuated by a control device (ECU) for controlling brake pressures. The electronic actuation is carried out in this case either depending on a driver's demand specified by a foot brake valve or else depending on an automatically specified setpoint vehicle deceleration. The axle modulators then pneumatically produce a corresponding brake pressure that is controlled at brake cylinders of the service brakes of the brake system.
Alternatively, the axle modulators can also be pneumatically actuated by the foot brake valve specifying a control pressure at the axle modulators corresponding to the driver's demand, depending on which the respective axle modulator produces the corresponding brake pressure for the service brakes. ABS functionality is integrated in both versions, so that locking of the wheels during braking can be prevented.
In addition, a pneumatic parking brake with a parking brake valve, which implements a parking brake force that is manually specified by the driver by actuating the parking brake valve, can be provided in a further parking brake braking circuit. In this case, the parking brake valve actuates a spring-loaded brake pneumatically depending on the specified parking brake force by means of a control valve, wherein the spring-loaded brake then holds the vehicle at a standstill. The parking brake can also be implemented as an electropneumatic parking brake, so that the parking brake force can also be specified automatically. By corresponding electrical actuation of the parking brake valve, the parking brake can then also be used as an auxiliary brake, in particular during operation of the utility vehicle, in order to bring about at least supportive braking. For example, a hill-holder function, a starting aid or an automatic parking brake can be implemented by this means.
It is a disadvantage of previous implementations of an automatically actuatable electropneumatic parking brake that many additional components are necessary for retrofitting a pneumatic parking brake that is already present in the vehicle with an electropneumatic parking brake, wherein said components are intended to enable electronic actuation of the parking brake. As a result, the costs and the fitting complexity during a retrofit are very high. But the costs of initial equipment are also increased because of the many components.
In EP 2 066 536 B1 it is proposed to provide a bypass valve implemented as a bistable surge valve in a trailer with an electronic brake system and a parking brake valve for manual actuation of the brake system by means of a spring-loaded brake. The bypass valve is changed over when the vehicle is switched off such that spring-loaded cylinders of the spring-loaded brake are vented and as a result movement of the trailer is prevented. This is intended to achieve that the trailer cannot be removed without authorization when parked. In order to also maintain the switching position of the bypass valve in the de-energized state, the bypass valve is implemented as a bistable surge valve, i.e. for a changeover the bypass valve is stimulated with a short current pulse; the switching position that is set as a result is also maintained in the de-energized state. It is a disadvantage in this case that the bistable surge valve is very cost-intensive.
In EP 2 190 706 B1 a valve arrangement for controlling a brake system of a trailer vehicle is described. According thereto, it is provided to adjust the parking brake braking pressure in a spring-loaded brake by means of a pneumatically controlled 3/2-way valve. If the vehicle is in operation, the 3/2-way valve is pneumatically changed over so that a reservoir pressure medium from a pressure medium reservoir of the trailer is introduced into the spring-loaded brake, i.e. a higher parking brake braking pressure that holds the spring-loaded brake in the released position is adjusted in the spring-loaded cylinders. If a parking function is activated by the driver, the 3/2-way valve is changed over such that a service brake braking pressure adjusted by the service brake function is set as the parking brake braking pressure in the spring-loaded brake. If the driver does not actuate the service brake when at a standstill, the spring-loaded brake is thus automatically applied. In the event of actuation of the service brake, the service brake braking pressure and thus also the parking brake braking pressure in the spring-loaded brake increase, so that the spring-loaded brake is released again. At the same time however, the rising service brake braking pressure is fed to the service brake cylinders, so that said cylinders are applied, and the vehicle is thereby held at a standstill. It is a disadvantage of this solution that very many additional components are necessary in order to retrofit an existing pneumatic parking brake, for example the actuation of the 3/2-way valve is carried out pneumatically, which additionally increases the costs of retrofitting. In addition, graduated manual specification of a parking brake force when the vehicle is in operation is not possible if the parking function is not set.
DE 35 06 418 C1 describes carrying out fully automatic actuation of the holding brake or the spring-loaded brake in a trailer in a spring-loaded holding brake and service brake device for the trailer if the spring-loaded brake in the towing vehicle is also activated. In addition, when the trailer is decoupled the spring-loaded brake is applied automatically. This is carried out by means of electrical actuation of a trailer brake valve that vents the spring-loaded cylinder of the spring-loaded brake once the control input of the trailer brake valve is de-energized. This is the case if the trailer is decoupled or a corresponding electrical signal for braking is specified by the driver, which is transmitted electrically both to a valve of the spring-loaded brake of the towing vehicle and to the trailer brake valve of the trailer. Braking by means of the two spring-loaded brakes is carried out depending on the degree of actuation of the spring-loaded brake, so that smooth actuation is enabled. Purely pneumatic actuation of the trailer control valve is not provided.
EP 2 055 541 B1 describes a driver assistance system with a bistable bypass valve and a monostable bypass valve. By means of the bistable bypass valve, a spring mechanism of a holding brake can be ventilated or vented by either the pressure medium reservoir for releasing the spring mechanism or a vent for applying the spring mechanism being connected to a pneumatic control input of an axle modulator with the bistable bypass valve in a corresponding a switching position. The monostable bypass valve is disposed between the bistable bypass valve and the axle modulator, wherein in one switching position the control pressure that is output by the bistable bypass valve is passed to the axle modulator and otherwise a flow connection is prevented. I.e., in the second switching position of the monostable bypass valve, the prevailing control pressure is held at the axle modulator.